Closure

限制變數在儲存環境的作用範圍

利用變數作用域的特性，加上一層function

避免污染global環境

變數不會在function執行完畢後消失

會保留結果至下次執行

code:without closure(javascript)

// without closure

var count = 0;

function counterTick() {

return ++count;

}

console.log(counterTick()); // 1

console.log(counterTick()); // 2

code:with closure(javascript)

// with closure

function counter() {

var count = 0;

function counterTick() {

return ++count;

}

return counterTick;

// also could be written as arrow function

// return () => ++count;

}

var countFunc = counter();

console.log(countFunc()); // 1

console.log(countFunc()); // 2

code:javascript

const a = [];

{

for (let i = 0; i < 10; i++) {

}

}

{

let i;

for (i = 0; i < 10; i++) {

}

}

{

for (let i = 0; i < 10; ) {

i++;

}

}